1. Field of the Invention
The present invention relates to a communication technique.
2. Description of the Background Art
In recent years, power line communication (PLC) using a power line, which is originally used for electric power supply, has been put to practical use. In the power line communication, a communication signal with a higher frequency than a commercial power supply frequency is superimposed on commercial electric power line, to perform communication.
However, since the power line communication is a communication system in which an electric power line connected with home electric appliances is used as a transmission media, communication quality may deteriorate under the influence of noise of the electric appliances (referred to as “home appliance noise”).
The influence of this home appliance noise increases in the vicinity of a peak where a commercial alternate current (AC) voltage waveform has a peak amplitude and hence, in order to avoid this, there has been proposed a technique of performing power line communication in a so-called vicinity of a zero crossing where the AC voltage waveform has zero amplitude (e.g. Japanese Patent Application Laid-Open No. 2009-284159).
In the case of transmitting and receiving data packets in the vicinity of the zero crossing, zero crossing timing is detected in which the commercial AC voltage waveform has zero amplitude, and based on the detected zero crossing timing, the timing of transmitting and receiving data packets is decided
However, since the accuracy in detection of the zero crossing timing is low, if the detected timing is shifted from the actual zero crossing timing, the communication performance may be affected by home appliance noise severely.
Furthermore, in the zero crossing timing, a cyclic phase change, which is called phase noise generated from a switching power supply or the like, may appear in the vicinity of the zero crossing timing, and when such a phase change occurs during a transmission period of a data packet, the data packet may be damaged and not be recovered properly by receivers.
Moreover, cyclic pulse noise in synchronization with the zero crossing timing may exist in the electric power line, and when communication is performed in the vicinity of the zero crossing, it may be affected by the pulse noise.